wayland: Allow configuring locale1 mode from kwinrc

KWin already supports following org.freedesktop.locale1 for the keyboard keymap, but only by passing a command-line option. This is typically used in SDDM when running the greeter in kwin_wayland.

We want to use locale1 mode in livesys environments too, so that environment-agnostic installers can configure the keyboard and also immediatly apply the same configuration to the running desktop. When KWin is started through startplasma we cannot easily supply an extra command-line option, so we need to make this configurable from the external environment, in the form of a KConfig.

The command-line option will keep precedence over the KConfig.

Closes: #223 (closed)